1. Field of the Invention
The present invention relates to building construction, and more specifically, to apparatus for anchoring shear walls to foundations and lower floors.
2. Background
Strong winds and earthquakes subject walls and others elements of a building to tremendous forces. If these forces are not distributed to the proper elements or structures capable of withstanding such force, the building may be torn apart. Foundations are often the strongest element of a building. Securely tying the walls of a building to the foundation greatly improves structural performance during periods of strong wind or earthquake. Securement promotes single body motion and limits whiplash amplification that often results in structural failure.
Under extreme conditions, a building may be violently loaded or shaken back and forth in a lateral (side to side) direction. If a shear wall is tightly restrained at its base, loads may be smoothly transferred to the foundation. The loads may then be resolved in the foundation, where they appear as tension and compression forces.
Buildings are often composed of long walls, (walls with a length greater than the height) and short walls (walls that have a length shorter than the height). The tendency for a wall to lift vertically off a foundation is inversely proportional to the length of the wall. Tall narrow shear walls, which may be found in nearly all homes, act as lever arms and may magnify an imposed load. In certain instances, the actual load on the securement system may be magnified to several times the originally imposed load.
The as-built building is generally not the building that will be sustaining loads induced by wind or by earthquake shaking. Wood components of the building structure, including floors, joists, sill plates, top plates, and studs, will shrink. Shrinkage varies greatly but ranges typically from about one-quarter inch under the best of conditions, to well over one inch depending on the total cross-grain stack up (depth) of wood.
Wall securement may prevent lateral and vertical motion between the walls and the foundation. Additionally, it may be necessary to support the wall against forces that would tend to distort the wall's general rectangular shape. Building codes often require external and load bearing walls to be shear resistant by providing a plywood plane to support shear forces that may be imposed on the wall. Many times, building codes also require lateral and vertical securement of a wall to the foundation. Lateral and vertical securement may be accomplished by employing hold-downs, also referred to as tie-downs.
Hold-down systems are employed to secure walls of upper levels to walls of lower levels, as well as walls to foundations. Again the principle is to secure the entire structure to the foundation where structural forces can best be resolved. However, lower levels can present amplification of structural weaknesses to upper levels. If a hold-down system installed on a given level cannot compensate for all shrinkage and crushing affecting that level, structural weaknesses may be amplified on adjacent levels. Hold-down systems need to be able to compensate for structural weaknesses throughout the structure, and not just within a given level.
Moreover, hold-down systems can be difficult to install and expensive to fabricate. Some hold-down systems require assembly within narrow tolerances, making assembly difficult and time consuming. Other hold-down systems cannot compensate for structural weaknesses throughout the structure, causing an overload of a hold-down system on a given level. Accordingly, a need exists for a hold-down system that may be easily installed and utilizes the full potential of the system over the entire structure. It would be a further advancement to provide a hold-down system that may be produced and installed in greater quantities with greater speed and less expense.